Process of preparing magnetic graphitic materials, and materials thereof

ABSTRACT

A process of preparing magnetic graphitic materials from graphite in a second container ( 3 ) that reacts with one of more transition metal oxide and in a first container ( 2 ) at a volume ratio of 1:1, in a closed reactor ( 1 ), heated up to a temperature between 600° C. and the melting temperature of the transition oxide (s) for 6 to 36 hours, under a pressure of 10 atmospheres with the help of a transfer inert gas through an inlet ( 5 ) and vacuum between 10 −2  torr to 10 −7  torr through an outlet ( 6 ), obtaining at the end of the process a graphitic material with long-lasting magnetic properties at room temperature. The material obtained exhibits a complex structure, with pores, bunches, pilings and edges of exposed graphenes and finds application in nanotechnology, magnetic images in medical science, applications in communication, electronics, sensors, even biosensors, catalysis or separation of magnetic materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.11/570,593, filed Oct. 5, 2007, allowed as U.S. Pat. No. 8,075,793,which is a national stage application filed under 35 U.S.C. 371 ofInternational Application No. PCT/BR2005/000110, filed Jun. 15, 2005,which claims priority from Brazilian patent application No. PI0402338-2filed Jun. 16, 2004.

FIELD OF THE INVENTION

The present invention relates to the field of magnetic graphiticmaterials, more specifically, to processes of preparing nanostructuralmaterials from commercial pure graphite and transition metal oxides inan inert atmosphere or vacuum and under heating.

BACKGROUND OF THE INVENTION

Nanostructural carbonous materials are being the focus of attention ofresearch, due to the potential commercial applications and the noveltyof their physical properties. The possibility of achieving properties ofinterest in macroscopic samples of carbon—such as long-lasting magneticproperties at room temperature—open a plethora of applications.

These materials may be used in magnetic imaging in medicine, or elseapplied in nanotechnology, communications, electronics, sensors, evenbiosensors, catalysis or separation of magnetic materials. However, formany years, the existence of pure carbon materials that could exhibitthis type of property was difficult to believe.

Existing processes that lead to the obtainment of microscopic amounts ofmagnetic carbon use nuclear techniques (proton bombardment) orconditions of extreme temperature and pressure that make them unfeasiblefrom the point of view of economy. In addition, they do not lead tomaterials having applicable ferromagnetic behavior when compared withthe background magnetic signal (which, in general, is stronglydiamagnetic).

In spite of the efforts for achieving magnetism in organic materials,very few systems proved to have this property. In the last few years,with the discovery of new allotropic forms of carbon, this field ofresearch has been reviewed by the discovery of ferromagnetism in thecharge transfer salt [TDAE]-C₆₀ and in polymerized fullerene, as citedby P. M. Allemand et al, Science 253, 301 (1991), T. Makarova et al.,Nature 413, 716 (2001) and R. A. Wood et al., J. Phys.: Condens. Matter14, L385 (2002).

In addition, some papers show the existence of magnetization loops ofthe ferromagnetic type in highly oriented pyrolytic graphite (HOPG), ascited by Y. Kopelevich, P. Esquinazi, J. H. S. Tones, S. Moethlecke, J.Low Temp. Phys. 119, 691 (2000) and P. Esquinazi et al., Phys. REv. B66, 24429 (2002).

Recently, two important papers showed, in a not ambiguous way, that theexistence of ferromagnetism in pure carbon is possible. One of thesepapers, by P. Turek et al, Chem. Phys. Lett. 180, 327 (1991) reports theinduction of magnetic orderings by proton irradiation on HOPG. Thismaterial shows magnetic ordering stable at room temperature.

Another paper reports the synthesis of a new allotropic form of carbon,a nanofoam totally consisting of carbon, which exhibits a behavior ofthe ferromagnetic type up to 9 OK, with a narrow histeresis curve andhigh saturation magnetization, see A. V. Rode, E. G. Gamaly, A. G.Christy, J. G. Fitz Gerald, S. T. Hyde, R. G. Elliman, B. Luther-Davies,A. I. Veinger, J. Androulakis, J. Giapintzakis, Nature (2004). Thismaterial was prepared by ablation of vitreous carbon in argon atmospherewith high-repetition and high-power laser.

Also U.S. Pat. No. 6,312,768 deals with this subject, describing amethod of depositing thin films of amorphous and crystallinenanostructures based on the deposition of ultra-rapid laser pulses.

However, despite the existing developments, there is still the need fora process of preparing magnetic graphitic materials in any amount,provided with long-lasting magnetic properties at room temperature, saidmaterials being prepared from graphite and transition metal oxides, bothpowdered and under reaction conditions that lead to the desired product.Such process and the associated graphitic product are described andclaimed in the present application.

SUMMARY OF THE INVENTION

Speaking in a broad way, the invention deals with a process of preparingmagnetic graphitic materials from pure graphite, said processcomprising:

a) providing a reactor with a first container containing pure graphiteand a second container containing one or more transition metal oxides,the graphite and the oxide(s) being finely divided, the containers beingplaced at great physical proximity, the volume ratio of graphite totransition metal oxide(s) being of about 1:1, the reaction system beingclosed, under pressure with values between high vacuum (10⁻⁷ torr) to 10atmospheres of an inert gas, and kept at temperatures between reactionbeginning temperature and the melting temperature of the transitionmetal oxide(s) for 6 to 36 hours, whereby:

-   -   i) the transition metal oxide, upon decomposing by action of the        temperature, generates a proportion of oxygen gas sufficient to        cause oxidative attach of the graphite and generating pores        therein; and    -   ii) the transition metal oxide is reduced, for the most part, to        the null oxidation state, whereas the carbonous material at the        end of the process presents two zones, the upper zone being        constituted by the desired product, of porous structure,        bunches, pilings, and edges of exposed graphenes;

b) at the end of the desired reaction time, recovering the graphiticmaterial with long-lasting magnetic properties at room temperature.

Thus, the invention foresees a process for obtaining magnetic graphiticmaterials at room temperature from pure graphite and one or moretransition metal oxides, said oxides being combined at any proportion,provided that the amount of graphite is in stoichiometric excess.

The invention also foresees a process for obtaining magnetic graphiticmaterials, the magnetism being detectable at room temperature, forexample by attraction to a permanent magnet.

The invention also foresees a process for obtaining magnetic graphiticmaterials at room temperature, said process being accessible forcommercial production without excessively sophisticated equipment ortechniques, the objectives of the invention requiring only standardreactors like furnaces at 1200° C.

The invention also foresees a material purely based on carbon, capableof exhibiting the cited magnetic properties at room temperature.

The invention further foresees a stable magnetic graphitic material,that is to say, a material that maintains its properties for prolongedtimes, at least for a few weeks.

The invention also foresees a magnetic graphic material in which therequired properties result from the topographic characteristicsintroduced in the original graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a scheme of the reactor used in the process of theinvention.

FIG. 2 illustrates an MFM-2D image and the corresponding 3D image, thetotal photographed area being of about 10 μm×10 μm, and wherein thewidth of each magnetic track is of about 1 micrometer.

FIG. 3 illustrates a SEM image of the magnetic graphitic materialobtained by the process of the invention.

FIG. 4 is a graph illustrating the magnetization curve (SQUID) vs.temperature, comparing the magnetic behavior of the material before andafter the process, for an applied external magnetic field of 0.01 T(1000 Oe).

FIG. 5 is a graph showing a detail of the magnetization curve (SQUID)vs. temperature, for an applied external magnetic field of 0.01 T (1000Oe), which shows the magnetic quality of the product obtained by theprocess of the invention. The insert shows a detail of the curve of theinverse of the magnetic susceptibility as a function of the temperature,and the determination of the Curie (Tc) temperature in approximately 200K.

FIG. 6 is a graph showing the magnetization curve (SQUID) vs. externalmagnetic field, showing the typical behavior of a ferromagnet exhibitedby the sample treated, in T=200K.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Therefore, the present process of obtaining a graphitic material havinglong-lasting magnetic properties at room temperature is basically anoxidative attack on pure graphite, originated from a proportion ofoxygen from the decomposition, at a temperature between that of thebeginning of the reaction of about 600° C. and the melting temperatureof the oxide or mixture of metal transition oxides in a closed systemand in the presence of a transfer inert gas.

Alternatively, the proportion of oxygen may be originated from oxygengas in amounts equivalent to those obtained from the decomposition ofthe oxide or mixture of transition metal oxides.

Pure graphite is useful for the present process, which is commerciallyavailable. In order to facilitate contacting graphite with the oxidizinggas from the decomposition of the transition metal oxide, pure powderedgraphite is used, which is held in a container inside a reactor inclosed system, as will be detailed hereinafter. Typically, and by way ofexample, the use of graphite with granulometry smaller than 0.1millimeters works adequately.

All the forms of pure graphite are useful for the process, as forexample, pyrolytic graphite, powdered pure graphite, or any othervariety or form of presentation of graphite.

The transition metal oxides useful for the process of the invention area metal oxide of the VII group, as Fe, Co and Ni; metals of the IBgroup, such as Cu and of the IIB groups, as Zn; metals of the IIIBgroup, as Sc; metals of the IVB, as Ti and of the VB group, as V; metalsof the VIB group, as Cr. A preferred transition metal oxide for thepurposes of the invention is cupric oxide (CuO), either pure or combinedwith other oxides at any proportion.

The transition metal oxide is also used in powdered form. The oxide ormixtures thereof are placed in the container without the need to takeprecautions with regard to compacting it. Since the reaction is in vaporphase, the oxide used should have sufficient pureness to ensure thatcontaminant components will not volatize, this being the onlyrequirement. Oxides of usual pureness used in laboratories work well.

Good results are achieved when a mixture of graphite with other elementsis used for the reaction. By way of example, the mixture of graphitewith boron up to about 10% by weight of boron produces the sought-formagnetic results.

According to the principles of the invention, the proportion of graphiteshould be stoichiometrically quite higher than that of transition metaloxide. However, if it is considered by volume, the volumes of thetransition-metal-oxide powder and pure graphite powder may be at theratio of 1:1 or very close to this, for better results.

The containers or melting pot used both for graphite and for metaloxide, and the furnace tube are built from alumina, without beinglimited to this material. Any material that bears process temperaturesis suitable.

In a preferred embodiment, the containers have approximately 8 cm inlength and 1 cm in width and height; the furnace tube has 2.5 cm indiameter and 96 cm in length. These dimensions are preferred forapplication on a work-bench scale, the values and the proportion betweenthem may be different for pilot scale or industrial scale.

The atmosphere of the closed system may consist of an inert gas used astransfer aid, which may be under pressures of up to 10 atmospheres, orelse, under vacuum. A typical inert gas is nitrogen or argon, in areasonable degree of pureness of, for example, 99.9%. No specialrecommendations are required for this gas in addition to the usualcommercial characteristics.

During the reaction, a vacuum is made in order to help in displacing thebalance of the reaction towards the products. The level of vacuum usefulfor the reaction is between the mechanical-pump vacuum (between 10⁻² and10⁻³ tor) and high vacuum (10⁻⁷ torr).

The range of temperatures at which the reaction takes place is from 600°C. (a range in which the oxidation reaction begins) to, for instance,the melting temperature of the transition metal oxide (or of the mixtureof transition metal oxides) chosen, or of this metal in the state ofzero oxidation. Typically, for cupric oxide, 1200° C. has proved to be avery suitable temperature.

The reaction between graphite and transition metal oxide occurs during afew hours, between 6 and 36 hours, with the preferred range between 14and 24 hours.

The process of the invention for preparing a magnetic graphitic materialmay be carried out in batches, as described, or alternatively in acontinuous manner. In this way, any type of reactor/furnace capable ofbearing high temperatures may be used.

According to the proposed process, the original graphite and thetransition metal oxides being in separated containers at the end of thereaction, the transition metals are completely or for the most partreduced to its state of zero oxidation. For better results, thecontainers holding graphite and at least one transition metal oxide arevery close to each other inside the reactor where the reaction takesplace to form magnetic graphite. According to the invention, theproximity between the containers holding the reactants ranges from 0 to100 mm, more preferably from 30 to 50 mm, and even mover preferably from5 to 15 mm.

In the container where originally there was pure graphite, carbonousmaterial is obtained, and it is possible to identify clearly twodifferent zones. The material in the upper zone has an amorphous aspect,opaque color, and that in the lower zone has a crystalline aspect.

The material having magnetic properties at room temperature of thepresent invention is that of the upper zone, which, studied by scanelectronic microscopy (SEM) and by atomic form microscopy (MFM), has acomplex structure with pores, bunches, pilings and edges of exposedgraphenes. On the contrary, the material in the lower zone does not havea magnetic behavior at room temperature. It is important to separate themagnetic and non-magnetic phases in order to achieve a purer andconcentrated material, which may be carefully made due to the clearlydifferent physical aspect of both phases, or else with the help of amagnet.

In addition to the mere detection of magnetism at room temperature bythe use of an ordinary magnet, magnetic force microscopy (MFM) has alsoshown there is a magnetic behavior in the material of the upper zone,thus showing the important role that the topographic characteristicsdescribed play to bring about these properties. Magnetic measurementsconfirm this intense magnetic behavior exhibited by the materialobtained by the process of the invention.

Without being linked to any particular theory, the behavior exhibited bythe magnetic product of the reaction may be attributed to theinteraction of orbitals sp³ and sp², and to the location of streams ofelectrons that were displaced before in the orbitals π of the graphenes.These electrons are forced to take a location, reaching the magneticmoment due to the microstructural variations introduced by thisprocedure. These electrons may form orbits located around the defectscreated. If, due to the relative arrangement in space between thematerial, many located orbits do not cancel each other, but rather addup, then they may produce a macroscopic magnetic moment, being capableof bringing about the effect described and claimed in the presentapplication.

Then SEM photograph illustrated in FIG. 3 clearly shows the propagationof pores through the different graphite blades, which is in accordancewith this explanation. In other words, the idea is that, if we havepores passing through various consecutive graphenes, the streams ofelectrons π that are located in the pores generate magnetic moments inthe form of spires over each other, in the form of a solenoidoverlapping its effects and giving a non-null net macroscopic magneticmoment.

The influence of metals for the existence of this magnetism has beendismissed by analysis of X-ray fluorescence and by dispersive energyspectroscopy (EDS), coupled to electronic scan microscopy. These studiesare carried out on the original graphite, without processing, and on themodified graphite, no difference being noted between the results.

The magnetic graphite at room temperature produced by the presentprocess is characterized by having complex microstructure, constitutedby pores that pass through various graphite blades—with diametersranging from a few nanometers to more than 1 πn and nano and microstructured forms with the aspect of bunches or pilings. The structure ofthe graphite obtained can be seen in FIG. 2.

The invention will now be described with reference to the attachedFigures.

FIG. 1 is a simplified diagram of the reactor used in the process of theinvention.

Basically the reactor (1) is a closed system, such as an hourglass,heated by a sleeve (4) or any heating device capable of supplyingtemperatures between 600° C. and the melting temperature of thetransition metal oxide (or mixture of oxides). Inside the reactor (1), afirst container (2) is placed containing the powdered transition metaloxide(s) described above, and very close to the first container (2), asecond container (3) is placed containing powdered commercial puregraphite, at a ratio by volume in the first and second containers (2)and (3) being of 1:1. Through an inlet (5) a transfer inert gas, forexample, nitrogen, is injected. Through the outlet (6), a vacuum is madein the system, which may vary from values obtained from mechanical pump(typically 10⁻²-10⁻³ torr) to high vacuum (10⁻⁷ torr).

When the system reaches temperatures suitable for generating oxygen gasfrom the transition metal oxide(s) contained in the first container (2),the oxidation of the graphitic material contained in the secondcontainer (3) begins and, consequently, the process of forming pores inthe graphite as well. Since the reaction takes place during 6 to 36hours, with a preferred period of time of 14 to 24 hours, the generationof pores in the powdered graphite may even produce spongy materials, ifso desired.

At the end of the reaction the graphitic material of the upper part ofthe second container (3) is recovered as a product of the reaction,exhibiting long-lasting magnetic properties at room temperature.

The yield in magnetic graphitic product material ranges from 1/10 to1/20 (by volume) of the graphite originally placed into the secondcontainer (3); by weight and by way of example, a reaction initiatedwith 5 grams of graphite produces approximately 0.25 grams of magneticgraphite.

FIG. 2 illustrates an image of magnetic force microscopy (MFM) of agraphite of the invention. The width of each magnetic track is of about1 micrometer. The figure enables the verifying of the degree ofstructuration of the product obtained. This structure degree enables oneto show that, at room temperature, the magnetic response of the materialis important and with clearly established domains.

FIG. 3 is a SEM photograph of a graphitic material of the invention. Inthis photograph it can be observed that the degradation of the graphenesthat gives rise to the pores mentioned before occurs successively inmore internal blades, reinforcing the described effect and causing theclaimed magnetic effect.

FIG. 4 is a graph of the magnetization curve (SQUID) vs temperature,comparing the magnetic behavior of the material before and after theprocess. FIG. 4 enables one to verify the enhanced magnetic modificationfound in the graphitic material by the treatment of the proposedprocess. The achieved modification is very clear and enables the totalreversion of the original diamagnetic bulk behavior of the graphite to avery intense ferromagnetic behavior. It enables one to infer this typeof such an important response may not be attributed at all to thepresence of impurities in the sample, since these impurities, ifpresent, would permit, at best, to observe a weak, undermined magnetism,only perceptible by subtracting the diamagnetic background of thegraphite bulk, which clearly does not happen in this case.

FIG. 5 is a graph that shows a detail of the magnetization curve (SQUID)vs. temperature, showing the magnetic quality of the graphitic materialobtained, as well as the Curie temperature of about 185K. The magneticbehavior of the material persists even at room temperature.

FIG. 6 is a graph that shows the magnetization curve (SQUID) vs.external field, showing that the graphitic product obtained by theprocess of the invention exhibits the typical behavior of a ferromagnet,at temperature of T=200K.

The above description proves, therefore, that it is possible to obtainmacroscopic amounts of a material with long-lasting magnetic propertiesat room temperature from commercial pure graphite and a transition metaloxide under relatively mild conditions of reaction and with easilyavailable equipment, and that the material thus obtained finds use inmultiple applications, as magnetic image in medical science, or elseapplications in communications, electronics, sensors, even biosensors,catalysis or separation of magnetic materials.

Therefore, the present application presents a highly competitive processfor obtaining magnetic carbon having physical properties hithertounknown.

1. A process of preparing magnetic graphitic materials, comprising thesteps of: a) providing a reactor with a second container containing amaterial comprising graphite and a first container containing at leastone transition metal oxide, the graphite and at least one transitionmetal oxide being finely divided, the first and second containers beingplaced in close physical proximity, wherein the reactor comprises aclosed reaction system under pressure with values from high vacuum (10⁻⁷torr) to 10 atmospheres, in the presence of an inert gas introducedthrough an inlet and vacuum made through an outlet, the reactor beingkept at temperatures between a reaction start temperature of about 600°C. and a melting temperature of at least one transition metal oxide withthe aid of heating devices, wherein: i) the transition metal oxide, upondecomposing by action of the temperature, generates a proportion ofoxygen gas sufficient to cause oxidative attack on the graphite andgenerating pores therein; and ii) the transition metal oxide is reducedto the null oxidation state, whereas the material in the secondcontainer at the end of the process exhibits two zones, an upper zonethereof being constituted by a carbonaceous material having an amorphousaspect and magnetic properties at room temperature, the carbonaceousmaterial having a structure having pores and edges of exposed graphenes,and a lower zone thereof being a non-magnetic phase; b) at the end ofthe desired reaction time, recovering the graphitic material with stablemagnetic properties at room temperature.
 2. A process according to claim1, wherein the graphite is commercial powdered pure graphite.
 3. Aprocess according to claim 1, wherein the graphite is pyrolytic.
 4. Aprocess according to claim 1, wherein the graphite is any variety orform of presentation of graphite.
 5. A process according to claim 1,wherein the granulometry of the graphite is smaller than 0.1 mm.
 6. Aprocess according to claim 1, wherein the graphite contains up to about10% by weight of boron.
 7. A process according to claim 1, wherein thetransition metal oxide comprises metals of the VIII group, including Fe,Co and Ni; metals of the IB group, including Cu and of the JIB group,including Zn; metals of the IIIB group, including Sc; metals of the IVBgroup, including Ti and of the VB group, including V; metals of the VIBgroup, including Cr, either pure or combined at any proportion.
 8. Aprocess according to claim 7, wherein the transition metal oxide iscupric oxide, either pure or in mixture with other transition metaloxides at any proportion.
 9. A process according to claim 1, wherein thereaction time ranges from 6 to 36 hours.
 10. A process according toclaim 1, wherein the reaction time ranges from 14 to 24 hours.
 11. Aprocess according to claim 1, wherein a yield of the magnetic graphiticmaterial is between 1/10 and 1/20 (by volume) of the graphite originallyplaced in the second container.
 12. A process according to claim 1,further comprising the step of recovering the magnetic graphiticmaterial the assistance of a magnet.
 13. A process according to claim 1,wherein a proportion of graphite is should stoichiometrically higherthan that of transition metal oxide.
 14. A process according to claim 1,wherein a distance between the first and second containers is from 0 to100 mm.
 15. A magnetic graphitic material comprising an upper zonethereof being constituted by a carbonaceous material having an amorphousaspect and magnetic properties at room temperature, wherein thecarbonaceous material has a structure having pores and edges of exposedgraphenes, and a lower zone being a non-magnetic phase.
 16. The magneticgraphitic material according to claim 15, wherein the magnetic graphiticmaterial has a complex microstructures, constituted by pores that passthrough various graphite blades, with diameters ranging from a fewnanometers to more than 1 μm and nano and micro-structured forms. 17.The magnetic graphitic material according to claim 15, wherein amagnetic force microscopy (MFM) image of a surface of the magneticgraphitic material includes a plurality of magnetic tracks on thesurface having a width of about 1 micrometer.